ENERGY SAVERS INTRODUCTION

Using radio-frequency (RF) to dry yam packages is often less expensive than using tradi- tional dryers. Even when the overall operating costs are the same or when RF drying is slightly more expensive, RF has other qualities attractive to potential users.

During RF drying, radio waves excite water molecules in textile packages and cause them to evaporate. Because yam molecules in the package absorb much less RF energy than water does, this method minimizes the danger of yel- lowing. Packages dry evenly and quickly, and the product has a softer hand and more level moisture content than packages dried with conven- tional means. There is also less dye migration.

This bulletin describes com- puter models created to pre- dict and compare the costs of drying with RF and conven- tional pressure dryers. (The model for the conventional pres- sure dryers is based upon infor- mation about Avesta pressure dryers, which are repre- sentative of other makes.) it also discusses an example of drying cotton packages with RF as compared to an Avesta dryer.

CREATING A MODEL To create its models, the In-

dustrial Electrotechnology Labo- ratory (IEL) undertook a two- year project that included case studies at 10 industrial sites.

These studies were used to determine the energy needed per pound of water to dry tex- tile materials with RF. The studies also determined the energy needed for centrifuging. The energy needs of an Avesta yam-package pressure dryer with a 1,000-pound capacity, the most popular conventional package dryer in use, were taken from published ‘articles for use in the model.’

All cost factors-labor, elec- tricity, spare parts, etc.-are included in the models for the RF dryer and the Avesta dryer and are used to predict total op- erating costs per pound of fiber. The models are shown with ex- ample calculations in Figures 2 and 3.

Note that energy needed to dry with RF is a function of pounds of water removed. Fi- bers of different types that contain the same amount of water to be removed will re- quire the same amount of en- ergy to dry with RF. Fibers that go in wetter or come out drier will have higher overall costs predicted by the model. Like- wise, energy use, and thus cost, will decrease for fibers, that require less moisture to be removed than the cotton used in the example.

A spreadsheet program for both of the models is available from IEL .~ Potential users may modify the models’ assumptions to reflect operating conditions and costs for any textile plant.

An example This example uses the

models to calculate the costs of drying 1,000 pounds of 100 percent-cotton-yarn packages to a final moisture content of 8 percent using either 1) a cen- trifuge and an RF dryer or 2) an Avesta pressure dryer with steam and water recovey. (For comparative purposes, costs for the Avesta actually will be given with and without water and steam recovery.) The RF dryer used for the example does not have a vacuum sys- tem. Note also that the base values in each model are standardized: labor rate, elec- tricity costs, and hours of op- eration are the same for each dryer.

Factors affecting costs IEL performed sensitivity

analyses on the computer

models to determine which of the many cost factors could sig- nificantly alter overall costs. A significant change is one that al- ters total costs more than 5 percent when the value of a factor is changed by 50 per- cent.

For the RF model, four fac- tors were significant: electric- ity, labor cost, the energy re- quired to remove water in the RF dryer, and handling time per package. Three factors were significant for the Avesta model: costs for steam, labor, and electricity. Thus, electric- ity and labor costs signifi- cantly affected both types.

The dryers have approxi- mately the same sensitivity to electricity in the example. RF drying, however, has much greater sensitivity to labor costs (Figure 1).

(continued on page 5)

5 6 7 8 9 10 I 1 12 13 I 4 15 Labor Cost C$/ hr ]

,,RF Dryer -Avesta Dryer

Cost (RF vs. Avesta) as Labor Cost Increases

Page 2

f

d

Example of Drying Cost of RF versus Avesta Dryers ($Ab dry fiber)

+------ AVESTA DRYER--------- + +-RF Dryer-+

COST Without Recovery With Recovery FACTOR (waterkteam) (waterhteam)

Utilities 0.0647 (86%) 0.0270 (72%) 0.0164 (50%)

Labor 0.0050 (07%) 0.0050 (14%) 0.0148 (45%)

Maintenance 0.0050 (07%) 0.0050 (14%) 0.0015 (05%)

TOTAL $0.0747 $0.0370 $0.0327

Results Overall drying costs are di-

vided into utilities, labor, and maintenance and are based on costs per pound of fiber. A comparison of the overall dry- ing costs for the example is giver, in Table 1 and shown in

t, Figure 4. In this example, RF drying costs 11 percent less than drying with an Avesta pressure dryer that has recov- ery of steam and water.

Costs to dry the lot with cen- trifuging and RF drying were $0.033 per pound of dry fiber. Costs to dry in the Avesta with- out heat recovery were $0.075 per pound of dry fiber and with heat recovery $0.037 per pound.

For the RF dryer, utilities are approximately 50 percent of the overall cost in the example and labor is 45 percent. For the Avesta pressure dryer, utilities account for most of the total cost: 72 percent with heat re- covery and 86 percent without heat recovery. Labor and main- tenance each constitute ap- proximately 14 percent of the total drying cost for the Avesta

l with heat recovery and approxi- mately 7 percent for the Avesta without heat recovery.

Every component of the util- ity costs was lower for the RF dryer than for the Avesta pres- sure dryer, with or without heat recovery. When the labor costs are added, however, RF drying and drying with an Avesta pres- sure dryer with recovery have comparable overall costs.

8.0 -

7.0 -

6.0 -

5.0 -

4 , O -

3.0 -

2.0 -

0.0 AVESTA AVESTA RF DRYER

C No Recovery) C W i t h Recovery)

U t I rt r es -Ma1 ntenance E l ~ a b o r

The RF dryer in this exam- ple costs $181,989, and the Av- esta costs $200;OOO.

RF advantages RF offers advantages over

conventional dryers aside from the economic considerations. Among these are

1) Improved quality of product 2) Uniform drying 3) Just-in-time processing 4) Versatility 5) Good working conditions

(cool work area, lint vented outside)

6) Operator safety (no hot parts on outside of machine)

CONCLUSION In this example, the cost of

using an RF dryer is slightly less than the cost of using the Avesta with heat recovery.

Even in situations when RF drying is slightly more expen- sive than conventional meth- ods, companies moving toward small lot sizes, multiple col- ors, and improved quality may find that the flexibility of RF ar- gues in its favor.

This is only one example, and results will vary under dif- ferent conditions, especially different costs for steam, elec- tricity, and labor.

NOTES 1. The energy consumption

is based on limited field measurements and Dawson, Charies H., Economic As- pects of Yarn Package Drying, Institute of Textile Technology Research Report number 347, October 25,1988.

2. Copies of the spread- sheet model can be obtained by writing Operations Man- ager, Industrial Elect rotech- nology Laboratory, Box 8301, Raleigh, NC 27695-8301 USA.

REFERENCES Material in this bulletin was

adapted from Cato, M.; Crab- tree, K.; Flora, D.; Grady, P.; Hodge, G.; and Mock, G.,

TILES: Assistance with Purchase Decisions and Optimization of Dryer Operation, Durham: North Carolina Alternative Energy Cor- poration, 199 1.

HANDBOOK OF RADIO FRE- QUENCY DRYING OF TEX-

ACKNOWLEDGEMENT The NCSU College of Textiles

would like to thank the North Carolina Alternative Energy Corporation for supporting the study described in this paper and the various cooperating industries across the state of North Carolina for allowing us to interrupt their daily work to make the measurements needed to document the studies.

IEL-ES-RF-3